A Question Regarding BQ7692000PWR

Hi Joy,

While there is no specific specification in the bq76920 datasheet, the function of the part expects that the BAT supply voltage is approximately the same as the VC5 input.  Power for internal operation comes from the BAT pin except for the bias for the input. REGSRC is also a power input with much higher currents to provide power to internal circuits as well as the REGOUT pin to operate the bq78350 or other circuitry.  The BAT supply current may be in the 100's of uA during temp sample while REGSRC supply current could be 10's of mA.

1. The part will have leakage paths from the inputs to the supply pin if the supply is missing. Operating the part without the BAT pin connected is not recommended, note the various schematics in the datasheet.  Current from VCn inputs to BAT is not recommended or charcterized.  In your system likely the leakage current from the inputs (C5 is most likely) flows through internal paths to power the part and out the BAT pin (10), then back into REGSRC pin (9) to power the remainder of the part and out REGOUT to power the bq78350-R1.  As the load changes with operation the voltage on pins 9 & 10 will vary.

2. Normally after a PF the monitor/AFE part (bq76920 here) would remain powered from the cell side of the fuse so that the BMS system can continue to report status to an outside system - the battery is expected to say it is bad.  To do this the bq76920 should be powered from the BATT+ side of the fuse. Referring to the EVM schematic, R3/D2 could come from BATT+ rather than C5. If your system also required that the BMS should power from BF1, a separate diode could feed the IC BAT pin filter also (EVM TP19 node).  That diode probably should not have a bleed resistor like R3, and you may consider whether R3 is appropriate for your system or if protection from transients might come from a component like D3.

3. After a fuse opens, BF1 should go to 0 V.  The fuse should be open, if the activation circuit remains on it should pull down the net depending on how the fuse opens.  If you have a dual-power supply to the BMS as mentioned in #2, be sure there is no leakage path to bypass the fuse.  Also check for leakage paths through your B+ to BF1 comparator circuit.  If there is a leakage path, be sure that the current is within acceptable limits for your system.

I hope this helps you to create a design appropriate for your system.